In addition to governing the release of thyrotropin and prolactin from the pituitary, TRH (L-pyroglutamyl-L-histidinyl-L-prolineamide) is known to exert a wide variety of effects on both the central nervous system (CNS) and the cardiovascular system (CVS). TRH has shown promise in the treatment of circulatory shock and/or CNS ischemic damage, as a promotor of the regeneration of injured spinal antidepressant and in the amelioration of symptoms associated with amylotrophic lateral sclerosis (ALS). Analogues in which histidine has been replaced by our new amino acids, 2-pyrrolealanine and 3-pyrrolealanine, have now been synthesized. These analogues are expected to confirm our hypothesis that the multiple TRH brain receptors fall into two classes, each binding a different tautomer of TRH. Thus, we expect that the pyrrole peptides, as stable analogues of the respective TRH tautomers, will each exhibit one or more of the separate activities of TRH. Two TRH analogues-4-NO2-Im-TRH and 2,4-I2-Im-TRH-have been found to improve behavioral recovery following traumatic brain injury in rats. Because 4-NO2-Im-TRH has little endocrine activity and 2,4-I2-Im-TRH has minimal cardiovascular effect, these results support the hypothesis that the neuroprotective actions of TRH and its analogues are independent of their endocrine or autonomic activities. Analogues of TRH have been prepared which contain thioamide moieties in the pyroglutamic acid ring, the carboxyamide proline terminues, and in both positions. The monothioamide analogues are comparable to TRH in their ability to release TSH in vivo or in vitro. The second analogue shows a higher affinity for pituitary receptors than TRH itself, while the former analogue had lower affinity and reduced selectivity. Thus, the subtle exchange of sulfur for oxygen can have a significant impact on both receptor selectivity and affinity for a bioactive peptide. Current efforts are devoted to the synthesis and evaluation of analogues of TRH designed to meet the requirements of our new concept of "ligand- activated affinity labels" (LAAL's). Ideally, such a compound would block a receptor site indefinitely.